The not-so-blue jays

Abstract

Watching birds is great entertainment, and there's fascinating physics behind how some get their colours, says Chanda Prescod-Weinstein

Chanda's week.

What I'm reading Now seems like either an excellent or terrible time to read Octavia E. Butler's Parable of the Sower.

What I'm watching I recently finished The Schouwendam 12, and I was very surprised by the ending.

What I'm working on I have two postgraduate students writing up their first papers on dark matter. Exciting!

HERE in New Hampshire, I live next to a wooded area, and covid-19 means that I have been spending a lot of time at home. This has translated into me becoming a bit of a birder.

All day long, I text friends with my sightings – a hawk walking on the ground hunting smaller birds, a female northern cardinal, a bird that I thought was a woodpecker but was actually a warbler. I enjoy all of the birds I see, but my favourite visitors are the blue jays. They have such a beautiful pattern of bright blue, black and white, and they are fast movers too.

Being the physicist that I am, I haven't just enjoyed the birds but have also been reading up on the science behind their colours. And here is the thing I learned: the blue jay's beautiful blue shades aren't real, they aren't blue like my jeans are blue.

Not only is this shocking revelation true, but the blue jay isn't special either. It is common for “blue” bird feathers to appear to be blue, without actually having any blue colour in them.

Reading this felt like saying that my hair isn't actually dark brown, it just looks that way. In fact, my hair looks the way it does thanks to melanin pigments, chemicals that absorb some parts of the light spectrum and reflect others.

Some of these, eumelanins, absorb light that doesn't look brown or black and reflect back the parts that do. This photochemical reaction is why most of my hair looks dark brown, and the loss of eumelanins is why increasingly my hair strands look white. Melanins, including red-yellow ones, are the same pigments that cause humans to have a diverse set of skin colours.

Melanin is also a source of pigments for bird feathers. The brown sparrows I see eating bugs on my lawn have feathers full of melanin. Those sparrows, just like us, look they way they do because of these pigments.

The blue jay, on the other hand, has a feather that has a special microstructure, and that microstructure mimics the photochemical reaction through a process called diffraction. This phenomenon is what occurs when light runs into a barrier, bends and has a tendency to spread out.

Blue jays are flying diffraction gratings, highly evolved, natural instruments of light manipulation

In the case of birds, at the microscopic level, some feathers are structured so that even though they are brown, when light interacts with them, only the blue parts of the light are reflected away to observers.

Reading about this as a physicist was pretty surprising because diffraction plays an important role in astronomy.

As I have mentioned in earlier columns, figuring out what's going on in the night sky is a bit of a detective game. We can't make the sky in our lab, so we have to get good at understanding what we are seeing and squeezing information out of the light that arrives here on Earth.

One of the questions we ask when looking at an object like another planet or a star is: what is it made of? One of the best ways to tell is by doing spectroscopy: looking at the spectrum of light coming from the object.

Based on lab experiments on Earth, we know which colours of light match with the presence of different chemical elements. For example, light with a wavelength of 589 change ok? I think it's 580 rather than 587. nanometres – orange – is sodium.

What has this got to do with bird feathers? The instruments that we use to look at this aspect of astrophysical sources, spectrographs, rely on a tool called a diffraction grating, which uses the phenomenon of diffraction to break the light up.

In some sense, then, the blue jays I have been watching are not just interesting birds of the corvidae family, distant cousins of the highly intelligent crow. They are also flying diffraction gratings, highly evolved, natural instruments of light manipulation.

Understanding this, I am now even more in awe of them than I was before.